Polymeric compositions are well-known and are used extensively as insulation materials in medical devices, as well as in wire and cable. Solid polymeric insulating materials are characterized by various physical and electrical properties which include resistance to mechanical cut-through, stress crack resistance, and resistance to dielectric failure.
Unfortunately, polymeric materials used as insulators in medium and high voltage environments often suffer from degradation process commonly known as “treeing,” which can ultimately lead to dielectric breakdown of the polymeric insulating material. Treeing is an electrical pre-breakdown process. As the polymeric composition breaks down, the damage progresses through the insulator, or dielectric, in a path that looks something like a tree. Treeing usually is a slow type failure and may take years to cause a failure in the insulation.
Trees that form in the presence of water, and in particular at low voltages, are called water trees. Electrochemical trees are similar to the water trees but are characterized by the presence of metal ions in the trees. When water is absent, the trees which form are called electrical trees. Treeing can occur and/or progress as a result of partial discharges, as a result of electrical impulses, AC or DC voltages, or in the presence of moisture. U.S. Pat. No. 4,206,260 contains a discussion of the electrical treeing problem in polymer compositions and cites numerous patents attempting to overcome this problem.
It is commonly believed that trees start at an imperfection in the polymeric material. This imperfection can be a small void or a small solid contaminant. Although efforts have been made to improve the manufacturing processes so as to reduce the incidence of imperfections, total success in eliminating structural faults not been achieved (P. Roseen et al., IEEE Transactions on Dielectrics and Electrical Insulation, 5:189-194 (1998)). Likewise, if the voids in the polymeric material are filled, there is slight improvement in resistance to treeing, but it remains a problem. As an example, U.S. Pat. No. 4,206,260 relates to insulation manufactured for high voltage power cable that contains an alcohol that imparts resistance to electrical tree growth to the composition.
Another avenue that has been explored to slow or halt progression toward dielectric breakdown in insulating polymeric materials involves the use of mobile voltage stabilizing additives (VSAs). A VSA typically contains an electron acceptor group and an electron donor group, positioned such that there is a potential for hydrogen bonding and reversible proton transfer between the two groups (see U.S. Pat. No. 3,445,394). VSAs are thought to trap and deactivate electrons, thus inhibiting treeing. Mobile VSAs are typically blended into the polymeric insulating material, and generally must be mobile and sufficiently compatible (soluble) with the material such they can migrate to the voids and solid impurities which are the points of treeing initiation. By filling and surrounding these faults in the insulating material mobile VSAs can retard the initiation of the trees; and by filling the tree channel as it is formed mobile VSAs can retard the growth of the trees. If a VSA is too mobile, however, it can migrate to the surface and evaporate or leach out of the insulating material. Leaching of the VSA into the surrounding environment results in less protection against dielectric breakdown and, in the case of implantable medical devices, for example, can release toxic agents into the body.
VSAs have also been added to silicone oils, thereby increasing their dielectric strength. Silicone oils find use as dielectric fluids in transformers and capacitors. In both of the described VSA uses, the additive is a small molecule capable of diffusion through the insulation matrix.
Many organic additives have been discovered which are quite effective in retarding the growth of both types of trees. Most of the voltage stabilizers are mobile aromatic compounds. Acetophenone is perhaps one of the best known anti-treeing agents; others include fluoranthene, pyrene, naphthalene, o-terphenyl, vinylnaphthalene, anthracene, alkylfluoranthenes and alkylpyrenes. A list of aromatic voltage stabilizing additives can be found in U.S. Pat. No. 3,445,394. Silanes and organosilanes can also function as VSAs (U.S. Pat. Nos. 3,984,338; 4,840,983; 4,492,647). In addition, water tree resistant compositions are disclosed in Japanese Patent Number Sho 56 [1981]-92946 (crosslinkable polyolefin resin composed of a silicone-grafted polyolefin, a di-t-butyl phenol derivative that acts as a water tree inhibitor, a free radical initiator and a silanol condensation catalyst), and Japanese Patent Number Sho 56 [1981]-109404 (power cable insulating composition composed of a polyethylene-polypropylene copolymer or a polyethylene-polypropylene-polydiene terpolymer, an organic peroxide, and a diorgano polysiloxane).
As medical devices evolve, they typically become smaller in size. Decreases in size for leads and other articles that make use of polymeric materials for electrical insulation are limited, however, because the potential for dielectric breakdown increases as the dielectric layer gets smaller or thinner. Inclusion of mobile VSAs in the polymeric material of these devices is problematic because the VSAs are more likely to diffuse or leach out of the thinner dielectric layers. A method for preventing dielectric breakdown of an insulating polymer that does not suffer from the disadvantages of mobile voltage stabilizing agents would therefore be a welcome advance in the field of polymeric electrical insulation in general, and to the field of electrically insulated medical devices in particular.